1. Field of the Invention
The invention relates to Ground Penetrating Radar (GPR), and more specifically to a GPR antenna array and timing circuit.
2. Description of the Related Art
Unlike upward-looking radar used for air traffic control and meteorology, the antenna array in a GPR is directed toward the ground. For example, GPR is used for geophysical applications such as mapping subsurface strata, locating toxic waste sites for remediation, and detecting of unexploded subsurface ordinance.
A GPR system comprises at least one transmitter that transmits an electromagnetic impulse, usually in the frequency range of 1 MHz to 10 GHz. The system also comprises at least one receiver that receives a reflected waveform. The length of the impulse is adjusted to match the desired frequency range. The desired impulse duration may be expressed in nanoseconds (ns) as 1/f, where f is a center frequency in Gigahertz (GHz). Therefore, a 1 GHz antenna is fed with an impulse of 1 ns duration, a 500 MHz antenna is fed with an impulse of 2 ns duration, and a 100 MHz antenna is fed with an impulse of 10 ns duration. Ideally, this gives the transmitted waves very broad frequency content, centered around the frequency f. In practice, the impulse is between one to two cycles of the frequency. Therefore, GPR systems are sometimes referred to as xe2x80x9cimpulsexe2x80x9d or xe2x80x9cultra-wide bandxe2x80x9d (xe2x80x9cUWBxe2x80x9d) radars.
Subsurface industries such as construction, utility location, environmental remediation, and unexploded-ordnance detection have long sought safe, reliable, cost-effective methods for xe2x80x9cseeing into the ground.xe2x80x9d The utility location market suffers greatly from inadequate location technologies that result in hundreds of millions of dollars in damages, delays, and lost revenue for utility companies and contractors every year, losses than can be reduced significantly by use of GPR. Three utility locating market segments, other than GPR, can be distinguished by their accuracy and price: (1) One Call; (2) private locating; and (3) subsurface utility engineering (SUE).
xe2x80x9cOne Callxe2x80x9d is a nationwide clearinghouse that provides an alert to all public and private utilities of when and where construction may impact their lines. By law, contractors must register their site with One Call, which in turn contacts all the relevant utilities so they can mark their utility lines. One Call locating systems are based on electromagnetic induction technology that sense current passing through a conductor attached to the underground utility. Utility companies, responding to a One Call work order, guarantee accuracy on conductive lines within twenty-four inches horizontally on either side, with no guarantee of depth. With One Call, utility line locations are simply painted temporarily on the ground, easily subject to erosion or destruction. This poor accuracy results in broken utility lines and revenue loss.
Construction, utility, and industrial companies often relay on xe2x80x9cprivate locating.xe2x80x9d Private locating provides a greater degree of accuracy than is delivered by One Call. These companies often hire a utility locating company or a geophysics company to apply more expensive and time-consuming locating techniques. Private locating companies typically use electromagnetic induction technology, GPR, and magnetometry. Often this includes excavation, the most reliable and expensive method for determining the exact location of utilities.
Industrial and utility companies, however, frequently require more accurate maps of the subsurface than One Call or private locating can provide. For instance, extra accuracy may be needed while excavating near an oil pipeline because it may be too dangerous to break the pipe. Or, it may be too costly to accidentally cut an interstate fiber optical cable carrying important communications. In such situations, excavators perform a total cost/value analysis, including consideration of risk/cost avoidance. Often, they are more willing to pay higher fees to ensure greater accuracy.
xe2x80x9cSUExe2x80x9d can provide more accuracy than One Call or private locating. SUE is a rapidly growing specialty service offered by geophysical and engineering companies. It entails planning and designing utility arrangements before highway or other larger infrastructure construction. SUE engineers painstakingly map all discernible utilities at a given site using a variety of traditional and advanced geophysical methods. SUE uses electromagnetic induction technology, GPR, and magnetometry. It is generally more costly than private locating services because it uses computer aided design to produce a permanent record of the location of utilities. Even this premium service often only identifies 80% of utilities with certainty, frequently less when unknown non-conductive utilities are present. Further, SUE is very expensive.
An advanced GPR system may overcome the disadvantages of One Call, private locating, and SUE by providing a cost effective method to locate and image conductive and non-conductive utilities, vertically and horizontally, with a margin of error to satisfy any excavating needs. An advanced GPR system may also provide a permanent record of images of the excavation site that can be used in the future.
There are technical difficulties that must be address to implement such a GPR system, however. As mentioned above, for instance, GPR, antennae may transmit an impulse signal that lasts for a very short time. Because the center frequency of a GPR system may exceed 10 MHz, there may be no xe2x80x9csampling circuitxe2x80x9d whose sampling and digitizing rate is fast enough to sample the whole received waveform at once with a high enough dynamic range. In order to solve this timing problem, it is common to transmit a plurality of impulses, each having the same waveform. Instead of sampling a received waveform multiple times, each of the plurality of received waveforms is sampled only once, but at a different point along the waveform. A signal processor acts upon these sampled points. It is very difficult, however, to accurately schedule the time when each transmitter transmits an impulse signal and when each receiver samples the received waveform. Typical GPR systems cannot accurately schedule the time when each transmitter transmits and when each receiver samples the received waveform in a way optimized for an antenna array.
Currently available systems capable of handling multiple antennas use one digitizing circuit (one A/D converter), and one impulse generation circuit. These systems thus select one pair of antennas and route the generated high-voltage impulse to the transmitter, and the received sampled, analog, value to the single A/D converter. Because of the difficulty of creating high-voltage impulses at a higher rate than approximately 100 KHz, and due to the limited speed of the existing A/D converters with sufficient dynamic range, the data acquisition rate is effectively limited to approximately 100-150 kHz, independent of the number of receiving and transmitting antennas used.
Furthermore, existing systems use combined receiving and transmitting antennas, without the possibility of individually positioning each antenna element to form a suitable antenna array. If the receiving and transmitting antennas are not separate, the array may not have suitable coverage or sufficiently different polarization schemes. Horn antennas may be separable, but may also be unsuitable for GPR applications.
Further, for a GPR system to be practical, it should easily fit onto a moving vehicle, trailer, or portable housing so that subsurface images can be formed as the system is moving. This requirement introduces width and length requirements on the shape, arrangement, and number of transmit and receive antenna.
Thus, there is a need for a highly accurate timing circuit capable of timing multiple transmit and receive antenna to accurately image the subsurface. Further, it is necessary to have a transmit and receive antennas that meet the necessary physical design constraints for a mobile system.
This summary and the following detailed description should not restrict the scope of the claimed invention. Both provide examples and explanations to enable others to practice the invention. The accompanying drawings, which form part of the detailed description, show several embodiments of the invention and, together with the description, explain the principles of the invention.
Methods and systems consistent with this invention control an impulse radar having a plurality of transmit antennas and a plurality of receive antennas, wherein a control circuit of the radar receives a transmit timing input signal and a receive timing input signal. Such methods and systems delay the transmit timing input signal and generate a number of intermediate transmit timing signals delayed with respect to each other by a delay time, select either the transmit timing input signal or a corresponding one of the intermediate transmit timing signals as a corresponding output transmit timing signal, delay the receive timing input signal and generate a number of intermediate receive timing signals delayed with respect to each other by the delay time, add the delay time to the intermediate receive timing signals, and select either the receive timing input signal or a corresponding one of the intermediate receive timing signals as a corresponding output receive timing signal.
A system consistent with this invention comprises an antenna array. Such an antenna array comprises a plurality of transmit antennas, a plurality of receive antennas, and means for selectively enabling the transmit and receive antennas to allow each of the receive antennas to receive energy from any one of the transmit antennas. In such a system, the plurality of transmit antennas may be linearly arranged, and the plurality of receive antennas may be linearly arranged and parallel to the transmit antennas.
A system consistent with this invention provides a high voltage generator and a high-voltage impulse generator for each transmit antenna, and a sampler and analog to digital converter for each receive antenna.